The present invention relates to surge arresters.
ZnO surge arresters, also designated varistors, are usually employed as voltage limiting elements in electrical circuits. This particular operation is well-known, as evidenced by the 1984 sales catalog of the H. Schaffner Industrieelektronik GmbH Co., Theodor-Rehbock-Str. 5, D-7500 Karlsruhe, Federal Republic of Germany. On page 59 of this catalog a filter circuit designated FN 35 2Z-10/03 for an alternating current electrical network consisting of a phase conductor, a neutral conductor and a ground conductor is shown. In this circuit the ZnO surge arresters are connected parallel to the so-called X capacitors with capacitances in the uF range (here 0.22 uF) between the phase conductor and the neutral conductor. They serve to limit voltage surges caused by differential mode disturbances occurring between the phase conductor and the neutral conductor. The known circuit provides no protection against voltage surges generated by common mode disturbances occurring between the phase conductor and the ground conductor or between the neutral conductor and the ground conductor as there are no surge arresters connected between these conductors. However, such an additional surge protection is necessary in NEMP (nuclear electromagnetic pulse) and EMC (electromagnetic compatibility) applications. In the known circuit merely so-called Y capacitors are present for transient filtering. For reasons of accidental contact protection these capacitors in contrast to the X capacitors have capacitances in the nF range (here 15 nF) only.
One object of the present invention is to provide increased protection against voltage surges in electric circuits containing capacitive elements having values of up to approximately 100 nF without increasing the cost of the circuit.